VIBRATIONAL SPECTRA OF ORGANOPHOSPHORUS COMPOUNDS
3815
Vibrational Spectra of Organophosphorus Compounds. 111. Infrared and Raman Spectra of (CH,),PSCI, (CH,),PSBr, and (CH,),POCI
by J. R. Durig, D. W. Wertz, B. R. Mitchell,' Department of Chemistry, Uniaersity of South Carolina, Columbia, South Carolina 899008
F. Block, and J. M. Greene Defense Research Department, U . S. Army Edgewood Arsenal, Edgewood Arsenal, Maryland (Receized April 3, 1967)
81010
The infrared spectra of liquid (CH&PSCl and (CH3)2PSBrhave been measured from 4000 to 33 cm-'. The infrared spectrum of (CH&POCl dissolved in various solvents has also been measured over the same frequency range. The Raman spectra of the sulfur-containing compounds in the liquid state have been recorded and depolarization values measured. The Raman spectrum of (CH&POCl was measured both on the pure solid and as a solution in benzene. Assignment of the fundamental vibrations based on depolarization data and position of the absorption bands is given and discussed. An alternative explanation is presented for the origin of one of the two bands of the proposed doublet for the P=S stretching vibration.
Introduction As part of our continued study of organophosphorus compounds,? the infrared and Raman spectra of dimethylphosphinoic chloride, dimethylphosphinothioic chloride, and dimethylphosphinothioic bromide have been recorded. Many conflicting assignments have appeared in the literature for the frequency of the stretching vibration of the P=S bond. Several alternative reasons have been put forth to explain the variations in the assigned frequencies for this normal mode. Recently, Chittenden and Thomas3 tabulated the characteristic infrared absorption frequencies of the P=S, P-S-(C), P-S-(P), P-S-H, and P=Se bonds, and two characteristic bands of widely different frequencies have been assigned as the P=S stretching vibrations. The range for the higher band was given as 685-862 cm-' and for the lower band 550-730 cm-'. In some cases the difference in frequency between the two bands is quoted as 150-170 cm-I. The most widely accepted suggestion for the origin of the two bands is that the bands correspond to different conformers of the molecule. However, the large variation in the frequency of these bands casts considerable doubt
on this explanation. I n order to shed more light on the subject, we have investigated the vibrational spectra of (CHs)2POCI, (CH3)2PSC1, and (CH3)2PSBr. This series of molecules cannot have different conformers, so if a second band is observed in the region of the P=S stretching mode, it must be assigned to a different motion in the molecule or a combination or overtone band. In order to be relatively certain as to the origin of the observed bands in the 550-860-cm-' region it was necessary to make complete vibrational assignments for each of the molecules. A comparison of the frequencies of the normal modes for this series of molecules with those found for the CHIPOF? and CH3PSClz molecules was expected to help explain the problem of accidental degeneracy for the bending modes of the X-P=Y group when X and Y are very similar in mass. ~~
~~
~~
~~~
~~~
(1) Taken in part from a thesis by B. R. Mitchell submitted to the Department of Chemistry in partial fulfillment for a Ph.D. degree. (2) (a) J. R Durig, F. Block, and I. W. Levin, Spedrochim. Acto, 21, 1105 (1965); (b) J. R. Durig, B. R. Mitchell, J. S. DiYorio, and F. Block, J Phys. Chem., 70, 3190 (1966). (3) R. A. Chittenden and L. C. Thomas, Spectrochim. Acta, 20, 1679 (1964).
Volume 71.Number 1.9 November 1967
3816
Experimental Section The infrared spectra were recorded from 4000 to 250 cm-l with a Perkin-Elmer Model 521 spectrophotometer. The atmospheric water vapor was removed from the spectrometer housing by flushing with dry nitrogen. In the high-frequency region, the instrument was calibrated in the usual manner4 while the lower region was calibrated by using atmospheric water vapor and the assignments of Randall, et aL5 The (CH&PSCl and (CH&PSBr spectra were recorded as contact films between CsBr windows while the spectrum of (CH&POCl was recorded as a CC14 solution contained in 0.1-mm CsBr liquid cells. Infrared spectra of approximately 10wt o/o solutionsin benzene were recorded in polyethylene cells from 300 to 33 cm-' with a Beckman Model IR-11 spectrophotometer. The instrument housing was purged with dry air and calibrated using water vapor and the assignments of Randall, et al.j All reported frequencies are expected to be accurate to better than *2 cm-l. The Raman spectra of the liquids were recorded on a Cary Model 81 Raman spectrophotometer which had a circulating filter solution of 125 ml of o-nitrotoluene and 1.75 g of ethyl violet in 3 1. of 2-propanol to isolate the 4358-A mercury excitation line. Depolarization values were measured by the method of Crawford and Honvitz.6 The Raman spectrum of (CHp)2PSBr was recorded using the normal 5-ml sample tube. A 0.6-ml sample tube was employed for the (CH&PSCI compound. The Raman spectrum of (CH3)2POCIwas obtained in a benzene solution. Owing to the weakness of the resulting spectrum, the polarization data on this molecule are only qualitative; i.e., the band was observed to be either polarized or depolarized only. The Raman effect on crystalline (CH&POCI was observed with a Cary-81 Raman spectrometer equipped with an He-Ne laser source. The laser work was not done in this laboratory, and, unfortunately, no data were taken above 1500 cm-'. The samples were prepared via the method of Pollart and H a r ~ o o d . ~ , ~ Results and Discussion All three molecules have a t most a plane of symmetry which passes through the X-P=S or X-P=O bonds. Thus, they all belong to the point group C,. Each molecule has 27 normal modes of vibration which can be broken down into 15 symmetric A' modes giving rise to polarized lines in the Raman effect and 12 A'' modes involving motions antisymmetric to the plane. Nine of the normal modes are due to motions of the skeletal part of the molecule while the remaining 18 fundamental vibrations arise from movements of the methyl groups. The Journal of Physical Chemistry
DURIG,WERTZ,MITCHELL, BLOCK,AND GREENE
As a result of the symmetry of the molecules being studied, all fundamentals of the methyl groups should give rise to both in-phase and out-of-phase vibrations of the two groups. However, because of the large mass of the phosphorus atom, the coupling is expected to be small and many of the vibrations are coincident in frequency. Dimethylphosphinoic Chloride. Dimethylphosphinoic chloride, (CH&POCI, should exhibit six CHI stretching motions, three of which are in-phase motions and belong to the A' symmetry species of the C, point group whereas the other three vibrations are out of phase and belong to the A" symmetry species. The three vibrations in each symmetry species can be directly correlated to the CH3 motions in a methyl halide of C3" symmetry. Thus, the four antisymmetric CH3 stretching modes are assigned to the band a t 2988 cm-' in the infrared spectrum of (CH&POCl (Figure 1). The lower frequency band, 2916 cm-l, is then assigned to the two symmetric CH3 stretching fundamentals. The infrared spectrum of the molecules also shows two peaks in the region of 1400 cm-l, the expected frequency of the antisymmetric deformations, and two peaks in the 1300-cm-' region, the frequency expected for the symmetric deformations. Thus, the bands at 1412 and 1400 cm-I are ascribed to the four antisymmetric deformations which have been split either by lowering of the symmetry or a coupling of the methyl groups, but not both. The 1303- and 1292cm-I bands are then assigned to the symmetric deformations, one in phase and the other out of phase. The methyl modes of lowest frequency, excluding the torsions, are the CH3rocking fundamentals. If a comparison of polarization data obtained on the (CH3)zPSCl molecule is made, there is little doubt that the 930and 906-cm-' bands are the in-phase CHI rockings. The out-of-phase CH3 rocking modes evidently do not couple as strongly as do the in-phase motions, for only one band, a t 865 cm-', is ascribable to both of these motions. The weak band a t 836 cm-' is too weak to be assigned to one of the rocking modes, and the only alternative is assignment as the v11 Y E = 855 cm-' combination band in Fermi resonance with the out-ofphase rocking modes. The phosphoryl (P=O) stretching fundamental is assigned to the strong band a t 1246 cm-I. This band
+
(4) "Table of Wavelengths for the Calibration of Infrared Spectrometers," Butterworth Inc., Washington, D. C., 1961. (5) H.M.Randall, D. M. Dennison, N . Ginsburg, and I. R. Weber, Phys. Rev., 5 2 , 160 (1937). (6)B. L. Crawford, Jr., and W. Horwitz, J . Chem. Phys., 15, 268 (1947). (7) K.A. Pollart and H . J. Harwood, J . Org. Chem., 27,4444 (1962). (8) K.A. Pollart and H. J. Harwood, ibid., 28, 3430 (1963).
VIBRATIONAL SPECTRA OF ORGANOPHOSPHORUS COMPOUNDS
3817
WAVELENGTH (MICRONS) 3
25
I
100
4000
,
3500
,
3ooo
,
,
, , 4, ,
2500
, , I , .
5
I
, I
2doo
, ,
ldbo
*
, 6I , , ,
I
, I ?,
, ,, , 7 .I , I, , ,
lboo 1400 FREQUENCY (CM')
,,
12W
,'?, ,
lob
,,I,,
,
!&&&&JJ3
8b
, ,,!,
600
,,?
A00
Figure 1. Infrared spectrum of a CCL solution of (CH&POCl in a 0.1-mm liquid cell equipped with CsBr plates. The regions of CClr absorption were rerun in a benzene solution, and the inlay in the frequency range of 700-1000 cm-' shows the result.
displays a very distinct shoulder at 1235 cm-' in the infrared spectrum, a phenomenon common to phosphoryl stretching vibrations. The question of phosphoryl doublets has been discussed by many research workers, and it has been concluded that the doublet arises from two conformers with different phosphoryl vibrational frequenciRss-16 or that one component of thedoublet has its origin in a molecular vibration not connectedwith the phosphoryl bond.sJ0J6 However, neither of these reasons is applicable to the dimethylphosphinoic chloride molecule because conformers cannot exist and there are no other fundamentals assignable to the region of 1250 cm-'. We have, therefore, assigned the 1235-cm-' band to the A' combination band vg v12 = 1240 cm-' which has obtained considerable intensity from a Fermi resonance with the phosphoryl stretching mode. In the Raman effect of the benzene solution of (CH3)2POC1,only one strongly polarized line appears in this region at 1243 cm-'. Our data are in good agreement with previous data on the effects of electronegativities on the phosphoryl stretching frequency. Thomas and Chittendenls 40 ;S*, give the empirical relationship VP-o = 930 where the summation is of the electronegativities over all the substituents on the phosphorus. The ?r constants given yield an expected v p , ~ of 1234 cm-' while the experimental value is slightly lower than 1246 cm-', but owing to the expected Fermi resonance, the exact frequency is not known. The Raman spectrum of crystalline (CH3)2POCIhas but one line in this frequency range at 1195 cm-'. Thus, the frequencyof the P=O stretching fundamental shifts abqut 50 cm-l in the crystal which indicates that a considerable amount of association in the crystal must be through the phosphoryl oxygen.
+
+
The infrared spectrum of (CH3)2POClhas two absorptions in the region expected for the PC2 stretching modes: a medium-intensity band a t 751 cm-l and a strong band a t 696 cm-'. The Raman effect shows a strong, polarized line at 700 cm-' and a weak depolarized line a t 760 cm-'. On this basis, we have assigned the higher frequency band to the antisymmetric PC2 stretching mode and the 700-cm-' band to the symmetric PC2 stretching vibration. The three remaining fundamentals of the mid-infrared spectrum can readily be assigned to the P-C1 stretching and the in- and out-of-plane O=P-Cl bending modes. Durig, et aL12' have shown that the P-Cl stretching mode is expected to lie in the region of 480 cm-I. Therefore, the strongly polarized Raman line at 487 cm-' is attributed to the P-C1 stretching motion. Polarization data on the two bands at 370 and 335 cm-I indicate that the higher frequency band should be assigned to the out-ofplane O=P-Cl bending mode while the weakly polarized band at 335 cm-I should be attributed to the A' bending motion. The reversal of intensities occurring in the infrared and Raman spectra is strong support for these assignments; ie., the A' bending mode is the (9)F.S.Mortimer, Spedrochim. Acta, 9, 270 (1957). (10) J. V.Bell, J. Heisler, H. Tannenbaum, and J. Goldenson,J . Am. Chem. SOC.,7 6 , 5185 (1954). (11) R. C. Gore, Discuasiona Faradav SOC.,9, 138 (1950). (12) L. J. Bellamy and L. Beecher, J . Chem. SOC.,475 (1952). (13) L. J. Bellamy and L. Beecher, ibid., 1701 (1952). (14) B. Holmstedt and L. Larsson, Acta Chem. S c a d . , 5 , 1179 (1951). (15) L.9. Maiants, E. M. Popov, and M. I. Kabachnik, Opt. Spedry. (USSR), 7, 108 (1959). (16) L. C.Thomas and R. A. Chittenden, Spectrochim. A d a , 20, 467 (1964).
Volume 71, Number 1.2 November 1967
DURIG, WERTZ,MITCHELL, BLOCK, AND GREENE
3818
1
300
I 280
I
1
I
260
240
220
FREQUENCY (CM")
Figure 2. Region of absorption of (CH&POCl in the far-infrared region. The spectrum was recorded in a benzene solution contained in I-mm polyethylene liquid cells. The starred band at 303 cm-l is the result of a weak absorption band in benzene.
stronger absorber in the Raman effect while the A" bending motion is the more intense absorber in the infrared spectrum. Examination of the far-infrared spectrum (below 300 cm-l) was expected to produce the five fundamentals not already assigned, i.e., the PC2 twisting, wagging, and deformation, and the in-phase and out-of-phase methyl torsions. The region from 33 to 300 cm-' produced only two additional bands (Figure 2): a very weak absorption a t 298 cm-l and a strong absorption a t 237 cm-l. These frequencies are much too high to be ascribed to the methyl torsions. The 298-cm-' band is extremely weak, but it could not be attributed to a combination or difference band of any of the assigned fundamentals. Consequently, we have tentatively assigned the 298-cm-' absorption to the PC2 wagging which preserves the symmetry plane. The Raman efThe Journal of Phyaical Chemistry
fect of crystalline (CH3)zPOCl shows two bands in the region of 237 cm-': one a t 258 cm-' and the other a t 221 cm-'. It would appear then that in solution the PCZtwisting and deformation are accidentally degenerate a t 237 cm-l, whereas in the solid the two bands are split by 17 cm-l. The higher frequency band is ascribed to the twisting mode whereas the lower band is thought to be due to the PC2 deformation. (See Table I for spectra of (CH3)2POCI.) Dimethylphosphinothioic Chloride. The motions of the methyl groups of (CH3)2PSC1are certainly expected to parallel those of the corresponding motions in the (CH3)2POClmolecule disscussed above. The infrared and Raman spectra (Figures 3 and 4) show the obvious similarities; consequently, no discussion is given for the assignments of the methyl modes for the (CH&PSCI molecule. The spectrum of (CH&PSCl has only three fundamental absorptions in the frequency region of 800-400 cm-l, but four bands are expected; the symmetric and antisymmetric PC2 stretching modes, the P=S stretching vibration, and the P-C1 stretching mode. The three bands are observed a t 750, 608, and 455 cm-' in the infrared spectrum. For CHaPSC12, Durig, et uZ.,~' have shown that the P=S stretching mode absorbs at 672 cm-'. The frequency of this mode in (CH&PSCl is thus expected to lie lower than 672 ~ m - ' . ~Thus, the 608-cm-' band is assigned to the P=S stretching vibration. The 750-cm-l absorption is ascribed to both PC2 stretching modes. The apparent accidental degeneracy of the PC2 stretching vibrations is believed to be the result of a mutual repulsion of the A' PC2 and P=S stretching energy levels. The result is that the symmetric PC2 stretching mode is pushed from its unperturbed frequency of about 700 cm-' (696 cm-' in (CH3)2POC1)to 750 cm-l, the frequency of the antisymmetric PC2 stretching fundamental. Such a perturbation would also lower the P=S stretching vibration which accounts for the rather low value of 608 cm-' found for the P=S stretching motion is this molecule. The remaining band a t 455 cm-l is confidently assigned to the P-C1 stretching vibration. The low region of the spectrum displays two bands in the infrared and three in the Raman. All the bands found are considerably higher than would be expected for the two methyl torsions. Thus, there are five modes which are expected to absorb energy in this region of the spectrum. The polarized band a t 276 cm-' in the Raman effect is assigned to the PC2 wagging of symmetry species A'. The band a t 217 cm-l is depolarized and is assigned to the A" fundamental described as a PC2 twisting motion. The third absorption is found a t 198 cm-1 and is polarized; it is attributed to the PC2
VIBRATIONAL SPECTRA OF
3819
ORGANOPHOSPHORUS COMPOUNDS
Table I : Infrared and Raman Spectra of (CH&POCI Infrared, cch soh, cm-1
2988 2916 1412 1400 1303 1292 1246 1235 930 906 865 836 751 696 488 367 334 2980 237"
Intensc
vw W
m sh
Raman, benzene soh (om-1)
lntensb
Depolarizationc
3000 2917 1409
30 100 30
dP P P
1243
15
P
Raman, crystal, cm-1
Assignments
S S
vs sh
1195
~7
934
S
P
14
P=O str
+
= 1240 cm-l in Fermi resonance with ~7 CH3 rock (in-phase) vu antisym CHs rock (in-phsse) vn, Y23 sym and antkym CH3 rock (out-of-phase) ~ 1 1 YZS = 855 cm-' in Fermi resonance with vn and V23 vZ4 antisym PCz str Y ~ Osym PCZ str ~ 1 P-Cl 1 str YZ5 O=P-Cl bend ~ 1 O=P-Cl % bend VI8 PCBwag V I 4 PCZ twist Y ~ PC2 E def Y15, v27 CHs torsions
vu
VIS
v8 sym
S S
+
W
m
760 700 487 370 335 297 234
S
S S
m m vw
...
dP P P dP P P dP
2 20 17 4 8 5 7
710 484 368 350 308 258 221
...
...
...
' Far-infrared spectrum was obtained in benzene solution. Intensity measurements in this sample are very rough owing to noise and fluorescence. Abbreviations used: s, m, w, v, p, dp, and sh denote strong, medium, weak, very, polarized, depolarized, and shoulder, respectively.
100
I-
z..m-/ -
-1
E60 z
tll
f.z
f
220-
4.-
m
I
1
I
35bo
3ooo
25bo
xxx)
~'~~~~~~~
1800
deformation vibration. Thus, the C1-P=S bending motions have not been assigned. The 276-cm-l band, however, is considerably more intense in the spectrum of (CH&PSCI than it is in the spectrum of (CH&POCl. It seems reasonable, therefore, to assume that this mode obtains a portion of its intensity from an accidentally degenerate in-plane C1-P=S bending mode which is
1600
1400
1200
loo0
800
600
400
:
0
also expected to give rise to a polarized Raman line. It is believed that the out-of-plane CI-P=S bending is not observed here owing to the similarity in the masses and bond lengths of the P-C1 and P=S bonds. Durig, et u Z . ~ ~ have , shown that the bending modes for the molecules CH3POF2and CHaPSC12behave as if the molecules had Cavsymmetry, and, consequently, only Volume 71. Number 19 November 1967
3820
DURIO,WERTZ,MITCHELL,BLOCK, AND GREENE
FREQUENCY (CM")
Figure 4.
Raman spectrum of 0.6 ml of liquid (CH&PSCl.
Table II: Infrared and Raman Spectra of (CH&PSCl" Infrared, liquid, cm-1
Raman, liquid, Intens
2990 2915
S
2255 2215 2155 2038 1408 1359 1299 1289 1225 954 915 857 750 678 608
W
S
Intens
iiation
Assignment
2987 2911 2786
49 100 2
dP 0.14 p 0.33 p
antkym CHI str sym CHI str 2 X (a,Y S , YIP, or V W ) = 2810 cm-1 ( ~ 4 , PS, YIP, or Y W ) ( Y Z ~or v u ) = 2263 cm-1 ( Y O or YN) V I = 2243 cm-1 ( ~ 4 , YS, YIO, or Y W ) vZ4 = 2155 om-1 ( Y E or VZI) Y24 = 2039 cm-1 ~ 4 ,us, YIO, YZO antisym CHI def YIO Y24 = 1358 cm-1 Y O , YZI s p CHa def
W
W S
W Ssh
W
i
S S
S S
1405
21
1294
2
dP
950 918 858 750
6 4 4 21
0.26 p
612 555 457 276 217 198
78 1 93 78
0.19 p 0.39 p 0.23 p 0.73 p ' / I dp 0.76 p
dp
"/i
S
196
S
S
*..
Y S , vi8
0.31 p ' / i dp '/idp
... *..
40
74
+ +
+
+
( v i 2 or VI*) = 12% cm-1 sym CHI rock (in-phase) YE antkym CHs rock (in-phase) m,VI s p and antisym rock (out-of-phase) Y O , Y24 sym and sntisym PCr str ? YIO P=S str 2 X ( V I Z or VIS) = 556 cm-l vi1 P - C I str ~ 1 2 , Y18 PCZwag, Cl-P=S bend Y26 PCP twist VI4 PC2 def YM Cl-P=S bend VIS, ~ z CHa i torsions VI
W S
PI, Y Z , io, Y I I
+ +
vw
455 278
...
Depolar-
cm-1
vi
" Abbreviations used aa in Table I. three of the five bending modes are observed. Analogously, the (CH&PSCI molecule would behave like a molecule with C2,. symmetry; the A" Cl-P=S bending mode is transformed to the A2 mode. An AS mode The Journal of Physical Chemistry
for a CzVmolecule is infrared inactive and is usually not observed in the Raman effect. An alternative explanation could be that the in- and out-of-plane Cl-P=S bending modes are degenerate since only a small split-
3821
VIBRATIONAL SPECTRA OF ORGANOPHOSPHORUS COMPOUNDS
loo
-
;;--------T/!;T\
Fg.60 bR O ki--
2I 40 -
-
'?! 1
v) -7
2 20 0-
/,
/
i I
I
I
I
I
I
I
I
1
I
I
I
I
K)
FREQUENCY CM
I
Figure 6. Raman spectrum of 5 ml of (CH&PSBr.
ting was found for the corresponding C1-P=O bending modes. (See Table I1 for spectra of (CH&PSCl.) Dimethylphosphinolhioic Bromide. In the case of the two chlorides just discussed, the perturbation resulting from lowering the symmetry from CaVto C, was not enough to split the degeneracy of the two antisymmetric CH3stretching modes. The infrared and Raman spectra of dimethylphosphinothioic bromide, (CH3)r PSBr (Figures 5 and 6), however, show a very definite shoulder on the high-frequency side of the band a t around 3000 cm-'. Both bands are depolarized and are located at 2990 and 2976 cm-l in the Raman spectrum. It would, therefore, appear that the introduction of the bromide atom onto the molecule was sufficient to remove the degeneracy of the two antisymmetric stretching fundamentals. The strong polarized band a t 2905 em-' is then assigned to the symmetric stretching. There is, however, still no coupling of the two methyl groups for these motions. Consequently, all three bands represent both one A' and an A" vibration which are coincident in frequency. In the Raman spectrum of (CH3)2PSBr,only one band
is found in the region of the methyl antisymmetric deformations. In the infrared spectrum, however, three bands are observed a t 1406, 1397, and 1389 em-'. If the splitting of the degeneracy or the coupling of the methyl groups were greater, one would expect to see four bands in this region. It is believed that the four absorptions are so close together that an accidental degeneracy occurs. The symmetric deformations are again split into the out-of-phase and the in-phase motions a t 1297 and 1286 cm-l. The infrared spectrum of the (CH3)*PSBr molecule shows four bands a t 954, 913, 895, and 861 em-'; the first two frequencies are represented by polarized Raman bands at 947 and 909 cm-', respectively, while the corresponding Raman frequency of the fourth band a t 848 ern-' is depolarized. No Raman counterpart to the 895-em-' band was observed. Using the same arguments as were used for the two previous molecules, we have assigned the 954-cm-' band to the in-phase symmetric CH3 rocking whereas the 913-cm-' band is ascribed to the inphase motion of the antisymmetric methyl rocking mode. The out-of-phase A" rocking vibrations are assigned a t 895 cm-1 for the symmetric mode and a t 861 em-' for the antisymmetric fundamental. The three remaining bands in the infrared spectrum are all due to skeletal modes. For reasons discussed in the previous section, the band a t 744 cm-l is assigned to both the A' and the A" PC2 stretching vibrations. The polarized band at 600 em-' is readily assigned as the P=S stretching mode whereas the other band, also strongly polarized, is ascribed to the P-Br stretching vibration of symmetry species A' and is found a t 368 cm-' in the infrared spectrum and at 373 ern-' in the Raman effect. The far-infrared and Raman spectra of (CH3)2F'SBr each contain four bands below 300 em-'. The bands Volume 7 1 9 Number 19 Xoaember 1967
3822
DURIG,WERTZ,MITCHELL, BLOCK, AND GREENE
Table III: Infrared and Raman Spectra of (CH3)oPSBra Infrared, liquid, cm-1
Intens
2982
m
2905
m
2240 2202 2140 2027 1415 1397 1389 1347 1297 1286 1199 954 913 895 86 1 852 746 601
W
368 263 239 201 168
S
... a
vw W W
sh m sh
Raman, liquid, om - 1
Intens
Depolarization
2990 2976 2905 2780 2560 2247 2205 2140
39 50 100 2 1 1 1 1
dp 0.10 p 0.18p 0.47 p ' / i dp 0.64 p ' / i dp
1393
12
' h dp
S
'/i
dp
1288
1
'/i
947 909
5 4
0.31 p 0.64 p
848
2
' h dp
744 600 544 373 264 235 202 163
18 63 2 98 33 79 16 97
' / r dP
dp
S
sh m sh S
S
S
8
W S
VI6
~ 2 v1i ,
1. I
J
+ +
+
antisym CH3 def
v5, VID, ~ 2 0
+
= 1344 cm-1
V~O
V24
Y6, V Z I
sym CH3 def
1200 cm-l sym CH3 rock (in-phase) vs antisym CH3 rock (in-phase) vzz sym CH3 rock (out-of-phase) vs antisym CHI rock (out-of-phase) VIO V ~ Z= 864 cm-l v g , V24 sym and antisym PCe str vlU P=S str VII V14 = 536 cm-l vil P-Br str VIZ PCZwag, v26 Br-P=S bend V13 Br-P=S bend VZ6 PC2 twist V14 PC2 def Y ~ Sv Z, ~CH3 torsions 2~10=
W S
antisym CHs str antisym CH3 str ~ 3 VU , sym CHa str 2 X ( ~ 4 , ~ 5 vie, ) or vao) = 2786 cm-1 2 X ( y e or V Z I ) = 2576 cm-1 ( ~ 4 ,~ 5 vie, , or ~ 2 0 ) V23 = 2241 cm-1 ( n o r v21) 3- v7 = 2235 cm-1 ( ~ 4 , VS, Vis, or V ~ O ) ( v g or V24) = 2137 cm-1 ( V 6 or ~ 2 1 ) V24 = 2032 cm-1 vil
'/i
W
sh
Assignment
...
0.13 p 0.40 p 0.23 p 0.70 p 0.50 p '/7 dp 0.78 p
v7
+
+
Abbreviations used as in Table I.
which are observed in the far-infrared region (Figure 7) are too high in frequency to possibly be ascribed to the two methyl torsions. In the spectrum of (CH3)2POC1 is was observed that the PC2 wagging mode was an extremely weak band, and this fact was used in assigning the strong band a t 278 cm-' in (CH3)2PSClto an accidental degeneracy of the PC2 wagging and the S=P-Cl bending modes. The spectrum of (CH3)tPSBr has a relatively strong absorption in this region, 263 cm-', which is again assigned to an accidental degeneracy of the PC2 wagging and S-P-Br bending modes. An alternative explanation is that the PC2wagging mode is simply too weak to be observed in the spectra of (CH3)2PSBrand (CH3)2PSC1,and the two strong bands may be ascribed simply to the X-P=S bending motions. The BrP=S in-plane bending is ascribed to the strong, polarized Raman line at 235 cm-'. The weak, depolarized band a t 202 cm-' is assigned to the PC2 twisting mode of symmetry species A". The final observed band is found a t 163 cm-' and is polarized; it is, therefore] The Journal of Physical Chemistry
assigned to the PC2 deformation which is symmetric to the plane of symmetry. (See Table I11 for spectra of (CH3)2PSBr.) Table IV is a summary of the fundamental frequencies of the three molecules studied in this work.
Summary One of the interesting observations in this study is the demonstrated coupling of the PC2 symmetric stretching vibration with the P=S stretching mode. In the spectrum of (CH3),POC1, two definite absorptions are attributable to the PC2 stretching modes: one a t 751 cm-' assigned to the antisymmetric vibration and the other at 696 cm-1 ascribed to the symmetric motion. However, in the spectra of (CH&PSCl and (CH3)9SBr only one band can be assigned to the stretching motions of the PC2 bonds, and these are found a t 750 and 746 cm-', respectively. The vibrational energy levels of (CH3)2POCI are thought to be unperturbed, and the two PC2 stretching motions ab-
3823
VIBRATIONAL SPECTRA OF ORGANOPHOSPHORUS COMPOUNDS
Table IV : Summary of the Fundamental Frequencies Description of mode
(CHs)zPOCl (CHi)pPSCl (CHahPSBr
A' Symmetry Species Antisym CH3str 2988 Antisym CH3 str 2988 Sym CHastr 2916 Antisym CH3 defa 1412 Antisym CH8 defa 1400 Sym CHs def 1303 P=O or P=S str 1246 Sym CH, rock 930 Antisym CH8 rock 906 696 PCz str P-X ( X = C1, Br) str 488 O=P-X or S=P-X 334 bending 298 PCZwag PC2 twist 237 CH3 torsion ... A" Symmetry Species Antisym CH3str 2988 Antisym CHI str 2988 Sym CH8 str 2916 Antisym CH3 def" 1412 Antisym CH3def" 1400 Sym CH3 def 1292 Sym CHI,rock 865 Antisym CH8 def 865 PCz str 751 O=P-X or S=P-X 367 bending PCz def 237 CHI torsion ...
2990 2990 2915 1408 1408 1299 608 954 915 750 455 278
2990b 2976b 2905 1415 1397 1297 601 954 913 746 368 239
278 217b
263 201
...
.*.
2990 2990 2915 1408 1408 1289 857 857 750
...
299d 2976b 2905 1389 1389 1286 895 861 746 263
196
168
...
FREOULNCY (CW?
Figure 7. Region of absorption of (CH&PSBr in the far-infrared region. The spectrum was recorded as a benzene solution in 1-mm polyethylene liquid cells.
730 em-'. The evidence that both bands arise from the P=S motion is the fact that both bands disappear during the isomerization of phosphorothionates to phosphorothioates
R
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7\
...
' The exact assignment of these frequencies to the four modes is unknown. Raman frequencies; all others are infrared frequencies.
sorb quite independently. In the case of (CH&PSCl and (CH&PSBr, however, the unperturbed vibrational energies of the symmetric PC2 stretching mode and P=S stretching motion are believed to be sufficiently close to one another to cause a strong repulsion of the two symmetric levels. The result is that the A' PC2 stretching fundamental is shifted to a higher frequency and covers the A" PC2 stretching mode. These conclusions are in agreement with the work of Hooge and Christen,"who state that the isolated P=S stretching frequency should be 675 em-'. Chittenden and Thomasa have done considerable work on the P=S stretching fundamentals, and they state that two bands arise from the P=S motion, similar to the P=O doublets discussed earlier. The two bands of the doublet, however, are split by some 150 em-', the over-all regions being 685-862 and 550-
-
S
RO
R
\/
7
OR
O
7\
RO
SR
This evidence seems extremely weak if one considers the fact that, upon isomerization, the P-C bond also disappears and, as mentioned in the text, that is the region expected for the PC stretching modes. Thus, it would appear that the disappearance of both bands upon isomerization is in excellent agreement with the assignments put forth in this paper. In conclusion, we believe many of the assignments for the P=S stretching vibration which were classified as band I by Chittenden and ThomasSare more appropriately described to other motions in the organophosphorus compounds. I n order to give more convincing data for two P=S stretching modes of vastly different frequencies, complete vibrational assignments are needed for the molecules in question. Acknowledgment. The authors gratefully acknowledge the financial support given this work by the National Science Foundation. (17) F. N. Hooge and P. J. Christen, Rec. Trav. Chim., 77, 911 (1958).
Volume 71, Number 18 Noiwmber 1967
